You know, for you were born then continues the divine speaker’s sarcasm of Job’s wisdom and is much like that of Eliphaz in 15.7. You know suggests “You know the answers to all these questions.” Born then means that Job was alive then, that he existed that long ago. Good News Translation has adapted verse 21a to refer to verse 20 by saying “I am sure you can.” The sarcastic reason Job supposedly can know is that he was already born at the time of creation.

And the number of your days is great translates the Hebrew literally. Logically this line is a consequence of the previous line. That is, since line a is true, line b is also true. “Because you were already born at the time of creation, you must be a very old man” or “You know all this because you were born when I created the world, and so you are very old.” These renderings may have to be restructured in some languages to bring out the irony; for example, “You think you know the answers as if you had been born when I created the world. You must certainly be a very old man.”

Quoted with permission from Reyburn, Wiliam. A Handbook on Job. (UBS Helps for Translators). New York: UBS, 1992. For this and other handbooks for translators see here .